1004 LIFE : OUTLINES OF GENERAL BIOLOGY 



"that metallic salts ingested with the larval food of affected species 

 might so act directly on their germ-plasm as to alter its potentialities". 

 And as smoke-deposits on the leaves of food-plants are found to 

 contain relatively large quantities of salts of manganese and other 

 metals, and so on hawthorn leaves in such districts also, careful 

 experiments were carried out by rearing larvae of successive genera- 

 tions upon such food-plants, with Mendelian carefulness of records, 

 and with due controls. The title of their Royal Society paper (1926) 

 — "The Induction of Melanism in the Lepidoptera, and its Subse- 

 quent Inheritance" — summarises their results; so their conclusions 

 may be briefly cited. First, that these "experiments demonstrate, 

 without possibility of contradiction, that the germ-plasm can be 

 influenced by external agencies. Irrespective of Lamarckian views 

 they supply what evolutionary theories all lack, an experimental 

 demonstration of at least one cause of variation: in fact they go 

 be3^ond this— and provide the principle, new in evolution, that food 

 not normal to a given organism may so affect its germ-plasm as to 

 give rise to heritable variations. This being granted, we see at once 

 how a change in habitat can originate new forms, and finally new 

 species. In no group of organisms would this be more potent than 

 in plants. In the variation of cultivated plants influences of the same 

 order are at work. Local races of animals find a ready explanation, 

 and so do the various forms into which wild animals break when 

 domesticated. In plant associations different species grow inter- 

 mingled, so what more likely than the accidental transference of 

 eggs or larVfC to the wrong food-plant ? Is not the difference between 

 species chemical? If larva? so transferred react as in the experimental 

 work, new phytophagous races or species must arise." This is claimed 

 as experimentall}^ proved for a gall-making saw-fly ; and as interpre- 

 tative of the difference of allied insect species on their distinct food 

 plants. 



The range of experiment thus opened will no doubt soon be 

 widely extended ; and it will be well worthy of attention on general 

 grounds as bringing the influence of nutritional environment as an 

 evolutionary factor in the origin of varieties and species to more 

 critical and comprehensive testing than ever before. Moreover, in 

 course of such investigations, with their controls, the often-alleged 

 influences of other environmental conditions may also be further 

 tested. 



Are there Modificational Species? — For many years in the 

 course of our j)ersonal studies the conviction has grown on us that, 

 in certain series of organisms, there are well-defined "species" which 

 differ from one another simply and directly because their environ- 

 ments are slightly different. We call them "modificational" species, 

 meaning that their differentiating features are not hereditary, but 

 are impressed on each successive generation as the direct results of 



